There exists a need for reliable sources of light in a short wavelength range commonly referred to as the “S-band” or “short-band” covering wavelengths between about 1425 nm and about 1525 nm. More specifically, what is required are wideband or broadband sources and narrowband sources of light in the S-band. Such sources are needed for various applications, including testing of optical components, measurements of optical system performance and generation of signals in the S-band.
In the field of testing, it is important to provide reliable S-band sources for examining the performance of filters, multiplexers, fiber Bragg gratings, circulators, isolators, arrayed waveguide gratings (AWGs), thin film filters, couplers, variable optical attenuators, detectors, receivers, amplifiers as well as various other active and passive optical components. Many of these components are used in telecommunications networks intended to operate in the S-band, where one of the important measurements is the insertion loss as a function of wavelength.
A common source of light is the tunable diode laser, which typically provides high power and high spectral resolution. Unfortunately, tunable diode lasers with suitable wide tuning ranges are costly. Tunable fiber lasers are also frequently used for testing applications because they too are able to provide high optical power over wide tuning ranges. Tunable fiber lasers tend to be somewhat lower cost than tunable diode lasers, though with only moderate (but often more than adequate) spectral resolution. A third type of optical source that is commonly used is an amplified spontaneous emission (ASE) or broadband source. ASE sources offer significantly lower cost, but with poorer spectral resolution. Since the ASE source produces emission over the entire wavelength range simultaneously, it only produces a small amount of power at each wavelength. Hence, the sensitivity and dynamic range of the measurements are limited. Also, the spectral resolution of the measurement is typically limited by the resolution of the optical spectrum analyzer that is used in conjunction with the ASE source. All of these types of optical sources are useful for various applications in addition to insertion loss measurements of telecommunications components.
In general, all of the above-mentioned types of optical sources are commercially available at wavelengths longer than those covered by the S-band, e.g., throughout the C-band and the L-band. The tunable diode laser is also available for the S-band, for example from about 1450 nm to about 1530 nm. However, tunable fiber lasers and ASE sources are not currently available in the S-band at costs comparable to those for the C-band or L-band. That is because of the lack of suitable S-band fiber amplifiers, e.g., Erbium-doped fiber amplifiers (EDFAs) or other suitable alternatives.
Specifically, in producing an EDFA for the S-band the relatively high losses and low gains over the S-band render the selection of fiber and the design of the amplifier system very difficult. In fact, the problems are so severe that the prior art teaches interposition of external filters between EDFA sections to produce an S-band EDFA. For example, Ishikawa et al. disclose a method of fabricating an S-band EDFA by cascading five stages of silica-based EDFA and four ASE suppressing filters in Ishikawa et al., “Novel 1500 nm-Band EDFA with discrete Raman Amplifier”, ECOC-2001, Post Deadline Paper. In Ishikawa et al.'s experimental setup, the length of each EDFA is 4.5 meters. The absorption of each suppressing filter at 1.53 μm is about 30 dB and the insertion losses of each suppressing filter at 1.48 μm and 0.98 μm are about 2 dB and 1 dB respectively. The pumping configuration is bi-directional, using a 0.98 μm wavelength to keep a high inversion of more than D≧0.7 (D, relative inversion). The forward and backward pumping powers are the same and the total pumping power is 480 mW. Ishikawa et al. show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
Yet another example of an approach using a number of filters at discrete locations in a wide band optical amplifier is taught by Srivastava et al. in U.S. Pat. No. 6,049,417. In this approach the amplifier employs a split-band architecture where the optical signal is split into several independent sub-bands, which then pass in parallel through separate branches of the optical amplifier. The amplification performance of each branch is optimized for the sub-band that traverses it.
Unfortunately, Ishikawa's and Srivastava's methods are relatively complicated and not cost-effective, as they require a number of filters. Specifically, in the case of Ishikawa five EDFAs, four ASE suppressing filters and high pump power are required. Also, each of the ASE suppressing filters used by either method introduces an additional insertion loss of 1-2 dB. The total additional insertion loss is thus about 4-8 dB.
Another approach to providing amplification in the S-band has focused on fiber amplifiers using Thulium as the lasing medium doped into a Fluoride fiber core (TDFAs). See, for example, “Gain-Shifted Dual-Wavelength-Pumped Thulium-Doped-Fiber Amplifier for WDM Signals in the 1.48-1.51-μm Wavelength Region” by Tadashi Kasamatsu, et. al., in IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001, pg. 31-33 and references therein. While good optical performance has been obtained using TDFAs, this performance has only been possible using complex, non-standard and/or expensive pumping schemes. Also, TDFAs suffer from the problems inherent to their Fluoride fiber host material, namely high fiber cost, poor reliability and difficulty splicing to standard silica fibers used elsewhere in the amplifier system.
Still other approaches to producing amplification systems based on rare-earth doped fiber amplifiers and cascaded amplifiers or pre-amplifiers followed by amplifiers are described in U.S. Pat. Nos. 5,867,305; 5,933,271 and 6,081,369 to Waarts et al. and in U.S. Pat. No. 5,696,782 to Harter et al. The teachings in these patents focus on deriving high peak power pulses at high energy levels. The amplifiers described in these patents are not suitable for producing broadband and narrowband sources for the S-band.
In view of the above, it would be an advance in the art to provide low-cost, reliable narrowband and broadband sources of light in the S-band. In particular, it would be an advance in the art to provide S-band sources that can be used for testing optical components, measuring the performance of optical components and generating signals in the S-band.